A thin film magnetic head is used as recording and reproducing means in a magnetic disk device. A prior art thin film magnetic head used for a magnetic disk device is illustrated in FIGS. 1A and 1B. FIG. 1A is a front view and FIG. 1B is a cross-sectional view taken along arrows A--A in FIG. 1A. This thin film magnetic head 1 includes a clean mirror-surface slider substrate 10 which is made of, e.g., an Al.sub.2 O.sub.3 --TiC system ceramic plate. On this substrate 10 is deposited, by sputtering or other conventional methods, an undercoat layer 12 made of SiO.sub.2, Al.sub.2 O.sub.3 or the like having a thickness of 10 and several microns. A lower core portion 14 is provided on the undercoat layer 12 by electroplating, for example. A magnetic gap layer 16 is deposited by a sputtering method or the like on the lower core portion 14 to form a magnetic gap 17. Like the undercoat layer 12, the magnetic gap layer 16 is made of non-magnetic material, for example, of SiO.sub.2, Al.sub.2 O.sub.3 or the like.
On the magnetic gap layer 16 is provided a conduction coil and insulating layer 18. A positive-type photoresist is normally used as the insulating layer 18 which is hardened to a stable state by heating. Conduction coils of respective layers are made of Cu or the like and formed to a thickness of several microns by electroplating. On the conduction coil and insulating layer 18 is provided an upper core portion 20 by electroplating. On the upper core portion 20 is provided, by sputtering, an insulating overcoat layer 22 which is made of SiO.sub.2, Al.sub.2 O.sub.3 or the like and which covers the entire upper core portion 20.
A pole portion 26 of the lower core portion 14 is opposed to a pole portion 24 of the upper core portion 20 through the magnetic gap layer 16 and foremost end portions 24a and 26a of the pole portions 24 and 26 are coplanar and opposed to a recording surface of a magnetic disk (not shown). The upper core portion 20 of the prior art thin film magnetic head has a generally fan-like shape in section as shown in FIG. 1A.
Core portions of a thin film magnetic head are generally formed by depositing a soft magnetic film in a magnetic field which is parallel to an axis of easy magnetization (hereinafter referred to as an easy axis). The easy axis is formed to be parallel to a recording surface of a magnetic medium. As a result, in the upper core portion 20 of the prior art thin film magnetic head, parallel magnetic domains 28 and triangular magnetic domains 30 as shown in FIG. 2 are formed.
According to the structure of the upper core portion 20 having such magnetic field structure, when the thin film magnetic head is driven by a recording current (a recording mode) or voltage due to a magnetic field produced by a magnetic recording medium (reproduction mode), magnetization changes in a direction shown by arrows A in FIG. 2 and this causes rotation of a magnetization vector in the parallel magnetic domains 28 and displacement of magnetic walls 32 by change of the magnetic domains in the triangular magnetic domains 30. In this case, the rotation of the magnetization vector (spin) in the parallel magnetic domains 28 responds quickly to the change in magnetization but the change in the magnetic domains in the triangular magnetic domains 30 which causes the displacement of the magnetic walls 32 occurs with some delay. This delay causes a waveform distortion called a wiggle during reproduction of a recorded signal. On the other hand, interaction between the magnetic domain walls 32 and outer side walls 34 of the upper core portion 20 causes discharge of distortion energy stored in the magnetic domains with some delay from the change in magnetization and this delay often causes a sharp pulse-like noise called a popcorn noise (noise-after-write) in a reproduced signal.
For reducing such wiggle waveform distortion and popcorn noise, it is conceivable to increase a core angle .theta. and thereby reduce the areas of the triangular magnetic domains 30 so as to reduce influence of the triangular magnetic domains 30. In the present invention, the core angle .theta. is defined as an angle made between foremost lines of the outer side walls of the upper core portion 20. Increase in the core angle .theta. however brings about increase in the entire area of the upper core portion 20 which in turn causes an increase in leakage of flux between the upper core portion 20 and the lower core portion 14 with a resulting decrease in recording and reproduction efficiencies. The increase in the area of the upper core portion 20 causes also an increase in inductance which causes an increase in noise. This is particularly the case with a thin film magnetic head of a multiple winding type.
Therefore, the inventors of the present invention have previously proposed a thin film magnetic head which is capable of reducing the wiggle waveform distortion and popcorn noise without increasing the area of the upper core portion. In this thin film magnetic head, a laminated structure is provided on a substrate including a lower core portion, a magnetic gap layer of non-magnetic material, a conduction coil, an insulating layer and an upper core portion, a back region which commences from the back end of a pole region of the upper core portion and has a top-forming portion formed on the top of the insulating layer having outer side wall portions which are substantially parallel to a center axis of the pole portion and having a larger lateral width than the pole region, and a transition portion which is located between the top-forming portion and the pole region formed. The transition portion is formed on a slanted surface of the insulating layer. A core angle .theta. at the transition portion is set between 120 degrees and 180 degrees.
By providing the outer side walls of the upper core portion in parallel to the center axis of the pole region in the above-described manner, the areas of the triangular magnetic domains increase and this would seem to cause an increase in the wiggle waveform distortion and popcorn noise. According to experiments made by the inventors, however, it has been found that the wiggle waveform distortion and popcorn noise are actually reduced by forming the upper core portion in the above-described manner.
An example of the thin film magnetic head is shown in FIGS. 3A and 3B. In these figures, FIG. 3A is a plan view and FIG. 3B is a cross-sectional view taken along arrows A--A in FIG. 3A.
This thin film magnetic head 36 includes a clean mirror-surface slider substrate 10 which is made of, e.g., an Al.sub.2 O.sub.3 --TiC system ceramic plate. On this substrate 10 is deposited, by sputtering or other conventional methods, an undercoat layer (not shown) of SiO.sub.2, Al.sub.2 O.sub.3 or the like having a thickness of 10 and several microns. A lower core portion 14 is provided on the undercoat layer by electroplating, for example. A magnetic gap layer 16 is deposited by sputtering or the like on the lower core portion 14 to form a magnetic gap 17. Like the undercoat layer, the magnetic gap layer 16 is made of non-magnetic material, for example, of SiO.sub.2, Al.sub.2 O.sub.3 or the like.
On the magnetic gap layer 16 are provided conduction coils 19 and an insulating layer 18. A positive-type photoresist is normally used as the insulating layer 18 which is hardened to a stable state by heating. Conduction coils 19 of respective layers are made of conductive material such as Cu or the like and formed to a thickness of several microns by electroplating. On the conduction coil 19 and insulating layer 18 is provided an upper core portion 38 by electroplating. On the upper core portion 38 is provided, by sputtering, an overcoat layer 22 which is made of SiO.sub.2, Al.sub.2 O.sub.3 of the like and covers the entire upper core portion 20.
A pole portion 26 of the lower core portion 14 is opposed to a pole portion 24 of the upper core portion 38 through the magnetic gap layer 16, and foremost end portions 24a and 26a of the pole portions 24 and 26 are disposed to be opposed to a recording surface of a magnetic disk (not shown). The upper core portion 38 and the lower core portion 14 are connected to each other at a rear gap through hole 40.
The upper core portion 38 has the pole portion 24 formed at a skirt portion 44 of the insulating layer 18 and a back region 42 which starts from the back end of the pole region 24 and is formed symmetrically with respect to a center axis 46 of the pole portion 24. The back region 42 has a top portion 42a formed on the top of the insulating layer 18 and a transition portion 42b. Outer side walls 48 and 50 of the top-forming portion 42a are formed in parallel to the center axis 46 of the pole portion 24. The transition portion 42b is located between the pole portion 24 and the top-forming portion 42a. A core angle .theta. of the upper core portion is set at an angle of 120 degrees or larger (preferably 120 degrees less than or equal to .theta. less than or equal to 180 degrees but it may be slightly larger than 180 degrees). In FIG. 3A, the core angle .theta. is determined at 180 degrees.
The upper core portion 38 is constructed as a two-layer structure of a first layer 38-1 and a second layer 38-2 laminated together. The first layer 38-1 is a core portion having no pole portion, in other words, solely consisting of a back region whereas the second layer core 38-2 is a core portion having both the pole portion 24 and a back region 42. The upper core portion 38 is made so that the back region 42 is formed to be thicker than the pole portion 24. This structure is adopted for preventing magnetic saturation in the back region 42 and thereby producing magnetic saturation in the vicinity of a throat height zero position THZ and also for reducing reluctance in the entire upper core portion. In the prior art structure, as shown in FIG. 3C which is a sectional view taken along arrows B--B in FIG. 3A, a foremost end surface 38a nearer to the pole portion 26 of the back region 42 in the upper core portion 38 consists of a first layer end surface 38-1a and a second layer end surface 38-2a which are located substantially at the same position.
The upper and lower core portions 38 and 14 are formed by depositing soft magnetic material films in a magnetic field which is parallel to an easy axis formed so as to form the easy axis which is parallel to a recording surface of a magnetic disk. As a result, as shown in FIG. 4, parallel magnetic domains 52 and triangular magnetic domains 54 are formed in the upper core portion 38. In this case, since magnetic walls 56a and 56b have the same length, displacement of the magnetic walls 56a and 56b becomes symmetrical when magnetization changes upwardly and downwardly as shown by arrows B in FIG. 4. As a result, storage of distortion energy is reduced whereby wiggle waveform distortion and popcorn noise are reduced. If the core angle is between 180 degrees and 120 degrees, generation of wiggle waveform distortion and popcorn noise due to the existence of the triangular magnetic domains having different lengths of the magnetic walls 56a and 56b is relatively small.
FIG. 5A shows results of measurement of wiggle waveform distortion and FIG. 5B shows results of measurement of popcorn noise when the core angle .theta. is varied. These results of measurement show that wiggle waveform distortion and popcorn noise can be reduced to a practically insignificant degree by setting the core angle .theta. at an angle of 120 degrees or larger.
When, in forming a back region of an upper core portion, a step portion is formed in a portion formed on a transition portion of an insulating layer and an overcoat layer is formed on top of the back region by sputtering, there often is produced a void (hollow portion or crevice) at this step portion.
FIGS. 6A to 6F show an example of a process of forming the upper core portion 38 in FIGS. 3A to 3C (illustrated with respect to the section taken along the arrows B--B in FIG. 3A). This formation process will be described below.